Approximately 400,000 people, including 104,000 children, had their drinking water cut off for four days this week after two filtration stations for the regional pipeline were destroyed by shelling, the UN International Children's Emergency Fund (UNICEF) said.
Urgent repairs were completed only Thursday evening, it added.
"Nearly three million people in eastern Ukraine rely on water infrastructure that is now in the line of fire," said Afshan Khan, the agency's regional director for Europe and Central Asia.
"We expect more families will be cut off from safe drinking water, putting children at severe risk of disease and other dangers."
The latest clashes between government troops and Russia-backed rebels reportedly killed ten Ukrainian soldiers over the past week, while schools, hospitals and basic infrastructure were seriously damaged.
In the rebels' de facto capital of Donetsk, power lines serving the city's water filtration station were hit earlier this month, threatening access to clean water for more than one million people, the report said.
Children who lose access to clean drinking water can quickly contract water-borne diseases including diarrhoea, UNICEF warned.
"All sides of the conflict must allow urgent repairs when water sources are destroyed and immediately stop the indiscriminate shelling of vital civilian infrastructure," Khan said.
More than 10,000 people have died and almost 24,000 have been injured since the pro-Russian insurgency began in April 2014, while 3.8 million people are in need of humanitarian assistance and more than 1.5 million have been forced from their homes.
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